Try Wolseley if you want a walk in option, they used to be Plumb Centre and Drain Centre and they still have depots that carry either drainage or plumbing. They are also very good if you just walk in with a photo or, in my case, a sketch of how my 3 story, multiple connection soil pipe was to be assembled. They are not the cheapest but it won't make much odds if you only require one part.
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